There exists a great need in the art for a simple, uncomplicated and readily executed procedure for applying a polymer coating or envelope around finely divided solid particles. While such a technique would be valuable in a variety of fields, its value is especially strong in the field of paint and similar coating formulations. Paint formulations separate basically into two types: The oil-based paint where the polymeric or resinous binder is dissolved in an organic solvent as a continuous phase with the pigment particles dispersed therein as discrete particles and a latex-based paint wherein the polymer or resinous binder exists as a dispersed phase or latex separately prepared by emulsion polymerization within an aqueous medium and paint pigment particles are dispersed within the aqueous medium as a dispersed phase separate and independent from the binder latex phase. Both formulations would, of course, typically contain a variety of other additives for various purposes, mostly unrelated to this invention. The latex-based paint has the advantage of lower costs and better odor since solvents are absent, and easy clean up by simple water washing instead of organic solvents, and it is in the context of latex-based paints that the present improvement finds especially advantageous application.
Although modern paints have been much improved in stability against settling or flocculation of dispersed material therein by means of various stabilizing additives and advanced dispersing techniques, latex paints are inherently subject to settling and flocculation with consequential undesirable effects upon their properties. For example, virtually any latex paint will undergo separation of the dispersed phase from the continuous phase if subjected to centrifugation even for a relatively short time. Such centrifugation represents an artifically exaggerated condition accelerating the unsuitable effects of gravity over longer periods of time. Flocculation and settling of the pigment phase are particularly undesirable since they lead to the clinging together of the packed pigment particles into agglomerates or clusters that tend to resist subsequent redispersion by agitation and degrade the hiding power of the resultant paint.
The opacifying capability or hiding power of a latex paint, or for that matter virtually any paint depends mainly upon three factors. First, light absorption due to the inherent coloration of the pigment particles, which is of minor importance, and then primarily only for tinted or nonwhite paints; second, light refractance which is fixed for any given combination of binder and pigment; and third, light reflection and dispersion or scattering by the surfaces of the pigment particles in the eventual solidified paint film. Every latex paint has a minimum filming temperature, characteristic of its particular binder and possibly other constituents, which is the minimum ambient temperature at which the discrete latex particles or globules in a film thereof will colaesce together during drying so as to result in a solid binder film. The binder film is in itself essentially colorless, or water white in color, and serves then as the vehicle for holding the pigment particles dispersed therethrough. The net hiding power is determined particularly in a white paint essentially by the scattering effectiveness of the thus dispersed pigment layer which is in turn highly influenced by the regularity of the arrangement of the pigment particles in the layer as well as the regularity of the particle sizes themselves. The latter can to some extent be contained within acceptable limits by proper control of grinding and dispersing techniques but the former is dependent virtually entirely upon the relative disposition of the myriad pigment particles throughout the solidified binder layer. If the pigment particles are uniformly spaced apart an ideal distance, their light scattering power will be optimum as will be the hiding coverage of the paint. On the other hand, if the particle spacing is irregular and if pigment agglomerates are present that deviate considerably from the desired uniform pigment size, the light scattering will be degraded as will the hiding power. Various ways have been attempted in the paint field to achieve an optimum physical disposition of the pigment particles within a paint film including the use of so-called extender pigments which essentially function as mechanical spacing elements for the opacifying pigment particles so as to thereby produce a uniformly and properly spaced pigment layer with optimum light scattering and hiding power. However, flocculation forces are particularly acute during colaescence of the binder latex upon drying, promoting the creation of irregular clumps and clusters despite the presence of extender pigments.
It will be apparent that if a solid polymeric coating or envelope could be applied with controllable generally uniform thickness around discrete separate particles of a finely divided solid, such as a paint pigment, such an envelope could act to precisely determine the spacing between contiguous particles in the ultimate paint film, provided that a dispersion of such uniformly enveloped particles was stable against settling, reagglomeration or coalescence of contiguous coated particles. Such coated particles would for purposes of paint formulation offer further advantages of great importance. For example, deterioration in color, particularly of tinted paints is largely caused by the attack of light and air upon the pigment particles within the dried latex film which normally exhibits some degree of porosity as to allow access of atmosphere to the pigment particles it contains. If, on the other hand, the pigment particles were completely enveloped within a continuous polymeric coating, they would be shielded against contact with atmospheric air and at least to some degree protected against the photolytic effect of sunlight due to the reflecting quality of the polymer coating. Thus, not only would a latex paint, wherein at least a significant portion of the pigment component thereof was united with at least a significant portion of the binder phase into a composite dispersed phase in which each particle formed a core within a uniform polymeric envelope, offer the advantage of substantially enhanced hiding or covering power, that power would be retained for a substantially longer period of time than with conventional latex paints.
Again, conventional latex paint films rarely exhibit the "scrub" resistance, i.e., resistance to abrasion, of a good oil-base paint since the necessity for escape of water from the film during drying creates inevitable channels or pores therethrough which constitute points of structural weakness and the coalescence and coagulation of the latex phase particles cannot lead to proper envelopment of the pigment particles without voids and spaces therebetween. If, on the other hand, each individual pigment particle were completely enclosed within an envelope of the binder polymer, total integration of the pigment into the dried paint film results upon drying with greatly increased scrub and abrasion resistance.
Furthermore, it is commonly recognized that conventional latex paint films are subject to staining, both in the sense of absorbing extraneous colored matter from the environment as well as allowing chalking or the migration of pigment particles onto adjacent unpainted surfaces, such as housing foundations. Staining is due primarily to the porosity of the film which allows the pigment particles to absorb extraneous colored matter while chalking is caused in part by porosity. If, however, the pigment particles were totally enveloped in a polymeric coating, solid paint film containing the same would necessarily exhibit very substantially reduced staining and chalking tendencies.
In addition, one of the most difficult operations in paint formulation is the effective dispersion of the pigment particles into the paint system, requiring expensive complex grinding and milling equipment together with a formulary of dispersant additives and stabilizers which add significantly to the overall cost of making paint. If it were possible to prepare pigmented dispersions, particularly already carrying polymeric envelopes, without the necessity for such elaborate and prolonged treatment, there would result a substantial decrease in the cost of paint manufacture and thus the ultimate cost to the paint consumer.
With these compelling advantages so clearly foreseeable, it is not surprising that many attempts have been made in the art to develop techniques for the polymeric encapsulation of finely divided solids such as paint pigments. The following prior art may be mentioned to illustrate such attempts:
U.S. Pat. No. 3,068,185--preliminary treatment of clay particles to sorb on at least the surfaces thereof free radical generating addition polymerization initiating agent, e.g., by exposing the clay under vacuum to a gaseous atmosphere containing the initiating agent, followed by admixture to a water suspension of the thus treated clay particles at least one addition polymerizable unsaturated monomer in amount up to about 30% by weight of the clay and heating the mixture to effect polymerization of the monomer;
Canadian Pat. No. 714,113--mixing a pigment, water and cationic surface active agent to render the pigment hydrophobic, plus an organic phase containing a polymerizable monomer, to cause transfer of the pigment from the water to the monomer phase, and then effecting the polymerization of the pigment-containing monomer while dispersed in an aqueous medium;
U.S. Pat. No. 3,544,500--water soluble polymer is preliminarily absorbed on the surface of solid particles which polymer either includes hydrophilic polymeric chains, e.g., in grafted form, or has associated therewith a surface active agent having one end adapted to be anchored to the adsorbed prepolymer layer with the other end providing a steric stabilizing effect around the particles, then a monomer which is a swelling agent or solvent for the preabsorbed polymer is added and caused to undergo polymerization;
U.S. Pat. No. 3,714,102--a cationic charge is established on an aqueous dispersion of finely divided solid particles by acidification of the medium and adsorption of multivalent aluminum cations from a compound releasing such cations present in amount to decrease the viscosity of the dispersion, then a polymerizable vinyl monomer is added and caused to undergo polymerization with the aid of a free radical polymerization initiator, the weight ratio of total monomer to solid not exceeding about 2.5:1;
U.S. Pat. No. 4,421,660--inorganic particles, e.g., pigment particles, are passed through a high shear mixing device, e.g., a Waring blender, homogenizer or ultrasonic mixer (col. 4, lines 38-43, Examples 1-6) and monomer is polymerized in the resulting dispersion forming a very low percent solids latex (less than 30%, as low as 7.4%) which is not suitable for paints or adhesives unless concentrated, e.g., by vacuum distillation. The resulting latexes also contain substantial amounts of particles having an average particle diameter which is only a fraction of the average particle diameter of the pigment thus indicating the presence of substantial amounts of pigment-less polymer particles. There is no disclosure or suggestion in the Solc patent of the use of substantially nonionic polymerization conditions, including the use of nonionic surfactants, particles (e.g., pigment) substantially free of ionic charges, monomer capable of forming a polymer free of ionically charged groups, and an initiating agent which is free of strong anionic groups and does not decompose to form such strong anionic groups. By contrast, the present invention does not require high shear mixing, produces high percent solids latexes, produces latexes containing predominantly polymer coated pigment particles with little or no pigment-less polymer particles, and utilizes substantially nonionic conditions at least in the early and intermediate stages of polymerization.
While each of these techniques might conceivably have accomplished their intended purpose, it is obvious that none of them is well suited for execution on a commercial scale. In particular, those techniques which involve the generation of cationic charges on the finely divided particles create serious practical difficulties. The resultant latexes cannot, for example, be mixed with conventional latex paints since if so combined, either deliberately or accidentally, flocculation can result as a consequence of the anionic nature of the conventional latex, dependent upon the relative degree of polarity of the two latexes. This problem can frequently be avoided by very careful mixing of the two latexes but in this case, the water sensitivity of the ultimate dried film will suffer. Also, cationic paint systems even when dried cause rusting of ferrous materials in contact therewith which precludes the application of such coatings over exterior surfaces of iron or even having exposed nails or other iron fasteners since otherwise rust would quickly develop. It, therefore, is perhaps not surprising that insofar as I am aware up to the present time, the goal of a simple, direct and effective polymer encapsulation technique for solid particles remains an elusive one for the paint and other industries.